1.1 Field of the Invention
The present invention relates generally to an optical element (OE) which can serve as an optical analogue to digital converter (OADC) and/or an optical digital to analogue converter (ODAC), and uses therefor. The OE can be usefully employed in a variety of applications to transform analogue information present in a light wave front into digital light signals and/or to transform digital information into analogue information in the form of the physical parameters of a light wave front.
1.2 Description of the Related Art
A discussion of art related to the present invention follows.
1.2.1 Interferometry
An interferometer is an instrument used to make measurements of beams of light. Interferometers measure properties such as length, surface irregularities, and index of refraction. The interferometer operates by dividing a beam of light into multiple beams traveling unequal paths. When these beams interfere with each other, an interference pattern is formed.
The interference pattern appears as a series of light and dark bands referred to as interference fringes. Information derived from fringe measurements can be used to make precise wavelength determinations to measure minute distances and thickness, to study spectrum lines, and to determine refractive indices of transparent materials.
A classic example of an interferometer is the Twyman-Green interferometer (a modification of the Michelson interferometer), which is used for testing optical elements such as lenses and prisms. The Twyman-Green interferometer uses a point source of monochromatic light at the focus of a quality lens. When the light is directed toward a perfect prism, it returns to a viewing point exactly as it was from the source, resulting in a uniform field of illumination; however, imperfections in the prism glass distort the wave front. Similarly, when the light is directed toward a lens backed by a convex mirror, it passes through the lens, strikes the mirror, and retraces its path through the lens to a viewing point. Imperfections in the lens result in visually observable fringe distortions.
Interferometers have the capacity to transform a light wave front, which varies according to the radius of the curvature of the wave front, into a series of light rings. The direction vector of the distribution of the wave front is, in effect, coded to the period and inclination of the stripes of the interferometer picture. See, for example, Max Born et al., Principles of Optics, (1968). The rings of the interferometer pattern are strictly related to the physical parameters of the incoming wave front. However, to the inventor""s knowledge, no one to date has used an interferometer to transform the information contained in a wave front into an optical digital image, such as an image consisting of a series of light spots.
1.2.2 Holography
Holography is a means of creating a unique photographic recording called a hologram. The recording appears to the naked eye as an unrecognizable pattern of stripes and whorls. However, when the hologram is illuminated by coherent light (e.g., a laser beam), the hologram projects a three-dimensional image of the recorded object.
While ordinary photographs record variations in the intensity (and sometimes color) of light reflected and scattered from an object, a hologram is a recording of both the intensity and phase of the light from the object. Holography depends on the effects of interferometry and diffraction to configure three-dimensional images.
A standard hologram is recorded as follows: A beam of coherent laser light is directed onto an object from a coherent light source. The beam is reflected, scattered, and diffracted by the physical features of the object and impacts on a photographic plate (this beam is referred to as the object beam). Simultaneously, part of the laser beam is split off as a reference beam and is reflected by a mirror onto the photographic plate.
The two parts of the laser beam-the reference and the object beams-meet on the plate and are recorded as interference fringes on the hologram. This pattern of fringes contains an optical record of the phase and amplitude of the light being reflected from the object.
To reconstruct the phase fronts of the object beam, the hologram must be illuminated by a beam which is similar to the beam used to construct the hologram. When the coherent light of the laser beam illuminates the hologram, most of the light from the laser passes through the film as a central beam. The fringes on the hologram act as a diffraction grating, bending or diffracting the remaining light to reverse the original condition of the coherent light waves that configured the hologram.
On the light source side of the hologram, a visually observable virtual image is formed. On the other side, a real image that can be photographed is formed. Both reconstituted images have a three-dimensional character because the hologram is a recording of both amplitude information and phase information. The phase information provides the three-dimensional characteristic of the image because it contains information regarding the contours of the object.
A common example of a hologram is the white-light hologram commonly used on a German visa. Such holograms permit an observer to view multiple images (typically about 10 images) depending on the angle from which the observer views the hologram.
Holograms are also commonly impressed on documents, such as credit cards, as a security measure, to display the control panels of aircraft on their windshields, and are used as an archival method for storage of experimental results, and to detect minute distortions in three-dimensional objects. See Laufer, Introduction to Optics and Lasers in Engineering, p. 204 (1996) (the entire disclosure of which is incorporated herein. by reference).
The present invention employs an OE, such as a specially recorded hologram, to transform varied incoming light into an output digital light code which can be visually observed and/or read by a photodetector and interpreted by a computer processor.
As will be discussed in greater detail below, the present invention also employs a variety of elements known in the art which can mimic the effects of a hologram, including for example, spatial light modulators and kinoforms. A kinoform is essentially a complex lens which operates on the phase of the incident light. The phase modulation of an object wave front is recorded as a surface-relief profile.
1.2.3 Use of Lasers in Distance Measurement
Lasers have been applied in a variety of ways to measure distance. Typical methods include interferometry, laser Doppler displacement, beam modulation telemetry, and pulse time of light.
Laser interferometers typically provide measurement of displacement from a starting position rather than a measurement of position. The instrument reading is typically set to zero as the initial position of the moving part, and the motion is then measured relative to this zero position. See Ready, Industrial Applications of Lasers, page 260 (1997) (the entire text of which is incorporated herein by reference).
Laser interferometry or distance measurement must be used in a controlled environment. Accordingly typical applications include setup of work holding fixtures for the production of aircraft engine components, checking out of the motion of machine tools, positioning operations, rack setup on boring mills, building vibration measurement, and measurement of strain in the earth. See Id. at pages 268 to 269.
Laser Doppler displacement distance measurement takes advantage of the Doppler shift of laser frequency effected when a stabilized laser is reflected from a moving surface. This frequency shift can be measured and converted to a measurement of surface displacement, i.e., the difference between a start position and position of an object.
Neither interferometric nor Doppler displacement methods can be used to measure large distances in uncontrolled environments. In particular, fluctuations of the density of the atmosphere over paths exceeding a few hundred feet make these methods impractical.
A common method of distance measurement used outdoors over long distances involves amplitude modulation of a laser beam and projection of the modulated beam toward a target. Distance is measured by comparing the phase of the modulated amplitude of returning light with the phase of the modulated amplitude of emitted light.
Pulse laser range finders are also commonly used to determine large distances. Commercial applications include generation of terrain maps, calibration of aircraft radar, and measurements of ranges of aircraft and ground vehicles. Pulse laser range finders emit a short pulse of laser light and measure the transit time for the pulse to reach a visible target, and for the reflected pulse to return to a receiver located near the laser.
The present invention employs an optical element, such as a hologram or kinoform, in a Range Finder to measure distance. The simplicity of the Range Finder of the present invention enables the production of low cost high quality Range Finders for a wide variety of uses, including measurement of microdistances, measurement in terms of meters and millimeters for construction and surveying, and measurement in terms of kilometers for large scale surveying and mapping applications.
1.2.4 Optical Computing
Digital signal processing and computing systems contain large numbers of interconnected gate switches and memory elements. In electronic systems the interconnections are made by use of conducting wires, coaxial cables, or conducting channels with semiconductor integrated circuits. Photonic interconnections may similarly be realized using optical wave guides with integrated optic couplers or fiber optic couplers and microlenses. Free space light beams may also be used for interconnections. Conventional optical components (mirrors, lenses, prisms, etc.) are used in numerous optical systems to establish optical interconnections, such as between points of the object and image planes of an imaging system. Computer processor-generated holograms, made of a large number of segments of phase gratings of different spatial frequencies and orientations, have also been used to configure high-density arrays of optical interconnections. A phase grating is a thin optical element having a complex amplitude transmittance. See Saleh et al. xe2x80x9cFundamentals of Photonicsxe2x80x9d pp. 855-857.(1991) (the entire disclosure of which is incorporated herein by reference.
Holograms are commonly used in optical switching and computing operations. Optical switches establish and release connections among transmission paths in a communication or signal processing system. (Id. p. 833). Holographic interconnection devices have been used to establish one-to-many or many-to-one interconnections (i.e., connecting one point to many points or vice versa). Id p. 859.
Optical elements can be fabricated by using computer processor-generated holography. This permits complex function to be encoded with the help of a binary function having only two values, such as 1 and 0 or 1 and xe2x88x921. The binary image is printed on a mask with the help of a computer processor. The mask plays the role of the hologram. The binary image may also be printed by etching grooves in the substrate which modulates the phase of an incident coherent wave, a technology known as surface relief holography.
Optical interconnections may be implemented within microelectronics using electronic optical transducers (light sources) acting as transmitters that beam the electric signal to an optical electronic transducer (photodiodes) acting as receivers. A reflection hologram can be used as routing device to redirect the emitted light beams to the appropriate photodetectors. Id. pp. 860-861.
Digital electronic computer processors are made from large numbers of interconnected logic gate switches and memory elements. Numbers are represented in binary systems and mathematical operations such as addition, subtraction and multiplication are reduced to a set of logic operations. Instructions are encoded in the form of sequences of binary numbers. The binary numbers xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are represented physically by two values of the electric potential. The system operation is controlled by a clock that governs the flow streams of binary digits in the form of electrical pulses. Interconnections between the gates and switches are typically local or via a bus into the operation is sequential (i.e., time is multiplexed).
A large number of points in two parallel planes can be optically connected by a large 3-dimensional network of free space global interconnections established using a custom-made hologram. For example, it is possible to have each of 104 points in an input plane interconnected to all 104 points of an output plane or each point of 106 points in the input plane connected to an arbitrarily selected set of 100 points among 106 points in the output plane.
The optical computer processor can be programmed or reconfigured by changing the interconnection hologram. Each gate can be connected locally or globally to a small or large number of other gates in accordance with a fixed wiring pattern representing, for example, arithmetic logic units, central processing units or instruction decoders. The machine can be programmed or reconfigured by changing the interconnection hologram. The gates typically are NOR gates. Each gate has two inputs and one output. The optical beam from each gate is directed by the hologram to the appropriate inputs of other gates. The electronic digital circuit is translated into a map of interconnections between the output and in points in the plane. The interconnection map is coded on a fixed computer processor-generated hologram. Data arrive in the form of a number of optical beams connected directly to appropriate gate inputs and a number of gates deliver the final output of the processor. The main technical difficulty preventing the creation of such a computer processor is the creation of large high-density arrays of fast optical gates that operate at sufficiently low switching energies and dissipate manageable powers.
Holograms are also used for simple operations in analogue optical processing.
However, while the foregoing operations in the optical computing field employ holograms to redirect beams of light, there is no direct correlation between the distribution of light in input plane and the distribution of light in the output plane, i.e. if any point in input plane is xe2x80x9cturned off,xe2x80x9d other points in the input plane are connected via the hologram to the corresponding point in output plane, such that the distribution of light in the output plane is not changed. Thus, information is transformed based on the intensity of light of every point in input plane separately from another points. For example, if many-to-one interconnections are present, the intensity of light of each point of the input plane is duplicated in the output plane such that if half of the points of the input plane are xe2x80x9cturned offxe2x80x9d then a corresponding light point is still present in the output plane. In other words, interconnection holograms do not transform the distribution of light in input plane as a whole, instead interconnection holograms transform light of every point in input plane separately from other points in input plane.
There is, therefore, a crucial need in the art for a means for using parameters of light other than intensity for carrying information, such as the radius of the wave front and/or the direction vector of the wave front. There is also a need in the art for a means of transforming information present in the distribution of the intensity of light of all points in an input plane as a whole.
In one aspect, the present invention provides an optical element (OE) having the capacity to transform information excluding intensity contained in a light wave front into digital information in the form of light shapes, such as spots, lines, etc. The optical element can be, for example, a specially manufactured hologram, computer generated hologram, or kinoform. In various aspects of the invention the information is contained in the radius of curvature and/or direction vector of the wave front, and is preferably not contained solely in the intensity of the wave front.
In another aspect, the present invention provides an optical analogue-to-digital converter (OADC) employing the foregoing OE. In another aspect, the present invention provides an optical digital-to-analogue converter (ODAC) employing the foregoing OE. The OADC and ODAC of the present are useful, for example in optoelectronic computing applications.
It yet another aspect, the present invention provides an instrument for measuring distance (referred to herein as a xe2x80x9cRange Finderxe2x80x9d) employing the OADC of the present invention.
Furthermore, the present invention provides a real-time holographic display unit employing the ODAC of the present invention.
Additionally, the present invention provides a device for measuring the parameters of light of wave fronts employing an OE of the present invention. Such a device can be useful, for example, in measuring the quality of optical elements, e.g., measuring wave aberrations of lenses and mirrors, and can be also used to replace or supplement a classical interferometric device.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications will become apparent to those skilled in the art from this detailed description.